Since mechanical properties of structural steel, turbine rotor steel, or the like, are associated with magnetic characteristics, the mechanical properties are evaluated by non-destructively measuring magnetic characteristics.
A coil used for a magnetic characteristic measuring device for measuring the magnetic characteristics is largely classified into a surface type probe measuring the change in a magnetic flux generated from a specimen by vertically disposing a coil on a surface of a specimen to be measured and an encircling type probe winding a coil around a round bar or a flake type specimen. The former is used to measure Barkhausen noise (BN) or harmonics and the latter is used to measure a magnetic hysteresis curve in addition to the BN or the harmonics. Among others, the former is usefully used for the magnetic evaluation of a non-destructively operated material.
An example of the reversible permeability measuring device was disclosed in paper “Nondestructive evaluation of aged 1Cr-1Mo-0.25V steel by harmonic analysis of induced voltage” (JMMM, Volume 231, 2001. 3) published in Journal of Magnetism and Magnetic Materials by the inventors.
The paper defines the correlation capable of measuring Vickers hardness, yield strength, and tensile strength, all of which are mechanical properties of artificially deteriorated 1Cr-1Mo-0.225V steel, non-destructively measuring reversible permeability that is a primary harmonic of an AC magnetic field with the surface type probe, and inferring mechanical properties with a coercive force that is ½ of a peak interval of reversible magnetic permeability.
A system of measuring reversible permeability with the surface type probe used in the paper was shown in FIG. 1.
The surface type probe is manufactured by winding a pick up coil to a yoke and then, winding an AC driving coil and a DC driving coil thereto. Meanwhile, a direct current is applied by power-amplifying a function generated from an I/O acquisition board with a power amplifier and an AC magnetic field applies a function generated from a function generator. The reversible permeability induced to the pick up coil is measured by a lock-in amplifier using a frequency of the AC magnetic field as a reference signal and values read by a shunt and the lock-in amplifier are measured through an input side of the I/O acquisition board and are then processed as data in a computer.
The magnetic characteristic measuring device having the above-mentioned structure uses a manner that obtains current from the voltage measured in the shunt to indirectly calculate a magnetic field H through the following Equation 1.
                    H        =                  ni          1                                    Equation        ⁢                                  ⁢        1            
(n: winding number per unit length, i: current obtained in shunt, and l: average magnetic path)
As represented by Equation 1, when the magnetic field H is calculated, it is calculated by an indirect calculation scheme, such that the measured value of the reversible permeability is inaccurate and the reliability of deterioration evaluation on materials is degraded.
The correlation capable of inferring the mechanical properties with a coercive force that is ½ of a peak interval of reversible magnetic permeability (PIRMP) using the magnetic characteristic measuring device having the above-mentioned structure is obtained, as shown in FIG. 2.
In this case, the measurement of the coercive force that is ½ of the peak interval of reversible magnetic permeability (PIRMP) is made as shown in a graph of FIG. 2. However, the graph shows result values that do not form a symmetry, such that the mechanical properties cannot be inferred accurately.